1. Field of the Invention
The present invention relates to an automatic transmission.
2. Description of the Related Art
The conventional automatic transmission provides parking (P), reverse (R), neutral (N), drive (D), second (S) and low (L) ranges so that it can change the vehicle speed automatically, responsive to selection of a range by the driver.
Specifically, when the driver selects a range, a control unit sets a gear stage, decides the shift and produces shifting output signals in accordance with the selected range, the vehicle speed and the throttle opening. Moreover, an oil pressure circuit actuates servo valves to apply/release frictional engagement elements corresponding to the set gear stage to thereby achieve the set gear stage in accordance with the shifting output signals of the control unit.
The rotation produced by the engine is transmitted through a fluid coupling to the transmission. In order to improve the mileage, however, the fluid coupling has a lockup clutch which engages when the vehicle speed reaches a predetermined value. Accordingly, if the speed at which the lockup clutch is to be engaged is set at a low value, the mileage can be correspondingly improved.
In the lockup diagram, therefore, a lockup area set for turning ON/OFF the lockup output is enlarged at the lower vehicle speed side.
In the ordinary automatic transmission, on the other hand, the selection of each range according to the intention of the driver is effected by moving the shift lever between the range positions arranged in a linear array. Thus, any one of the range positions arrayed in the so-called "I" line can be selected.
In this case, however, the gear stages cannot be freely and manually selected with a sufficient feel of a manual shift. Thus, there is provided a shift pattern in which the shift positions are arranged in the so-called "H-letter shape" array, as in the manual transmission. The "H-letter shape array is added to the range positions of the "I" pattern array, as used in the automatic transmission of the prior art, so that a range can be selected from those in the automatic transmission mode and in the manual transmission mode (as disclosed in Japanese Patent Laid-Open No. 157855/1986).
In order to achieve the feel of operation of a manual transmission, the manual transmission mode provides for operation of the lockup clutch over a wide range. Accordingly, lockup diagrams are provided which can be individually referred to in the automatic transmission mode and in the manual transmission mode, and the lockup area in the lockup diagram for the manual transmission mode is enlarged to the lower vehicle speed side.
In the automatic transmission of the prior art described above, however, enlargement of the lockup area in the lockup diagram over a wide range of engine rpm results in shock if the lockup clutch is locked up while the vehicle is running within some gear stage. As a result, not only may the driver be confused into thinking that the shift has been effected when the lockup clutch is locked up, but also driving comfort may be disturbed by an unexpected shock.